Method and apparatus for balancing propellers



Jan. 4, 1944. MARTlN 2,338,293

METHOD AND APPARATUS FOR BALANCING PROPELLERS Fi led Sept. 27, 1941INVENTOR EPJEMEFZJh BY 7W AT TO R NEY Patented Jan. 4,1944

UNITED. STATES PATENT] OFFICE METHOD AND APPARATUS FOR BALANG PROPELLERSErie Martin, West Hartford, Conn., assignor to United AircraftCorporation, East Hartford, Conn., a corporation of Delaware ApplicationSeptember 21, 1941, Serial No. 412,655

7 Claims.

This invention relates to an improved method and apparatus for balancingrotating bodies to eliminate vibrational effects therefrom and hasparticular reference to an improved method and apparatus for balancing arotating body having aerodynamic characteristics, such as anaeronautical propeller.

An object of the invention resides in the provision of an improvedmethod and apparatus for separately measuring the mass and aerodynamicvibration producing characteristics of such a body so that these defectscan be independently cor rected to provide a body'having substantiallyIn the accompanying drawing, in which like reference numerals are usedto designate similar parts throughout, there is diagrammaticallyillustrated a suitable apparatus for'the purpose of disclosing theinvention. The drawing, however, is for the purpose of illustration onlyand is not to be taken as limiting or restricting the invention since itwill be apparent to those s in the art that various changes in theillustrated arrangement may be resorted to without in any way exceedingthe scope of the invention.

In the accompanying drawing, the single figure isa diagrammaticillustration of an apparatus for separately detecting and measuring theeffects produced by aerodynamic and dynamic unbalance of a rotatablebody such as an aeronautical propeller.

In the manufacture of aeronautical propellers according to modernpractice, methods have been developed for balancing the propellers bothstatically and dynamically toa high degree of precision to eliminatepropeller induced vibrations in aircraft power plants. It has beenfound. however, that an aeronautical propeller which is perfectlybalanced mechanically during manufacture so that it does not cause anyappreciable vibration at low altitudes may become the source. ofundesirable vibrations when placed on an airany one altitude butfwhenthe altitude, and consequent air density, is varied, the aerodynamicforces vary in proportion while the mass distribution forces remainsubstantially constant so that a mass distribution which counterbalancesan aerodynamic defect at one altitude does not have that effect atadlflerent altitude. In order to overcome this difliculty it, therefore,becomes necessary to provide the propeller initially with substantiallyperfect mass and aerodynamic balance. Sincethe eflectof mass andaerodynamic unbalance occur at the same time and have substantially thesame or at least overlapping amplitudes, in order to correct both ofthese defects it becomes necessary to provide some means for separatelyindicating the defects quantitatively and positionally audit is amongthe objects of this invention to provide such a means'together' with anillustrative method of using such means to accomplish the resultindicated.

Referring to the drawing-in detail, the numeral It generally indicates apropeller the defects of which are to be measured and corrected. Thispropeller is mounted upon ashaft l2 driven by a motor ll through asuitable driving connection, as indicated by the gears II and II. Theshaft i2 is supported in special bearings, as indicated killed at IIIand 22, supported for freedom of lateral, movement by suitable means,such as the flexible suspension member I4 extending from the bearingtorigid anchor members I! which are in turn supported ona suitableplatform or machine base 28. In the arrangement illustrated'in whichwhile the bearing 2!! remote from the propeller would be arrangedtocarry an upwardly directed load. These bearings may be of the type shownin the Glsholt Machine Company catalog referred to below. Suitablemeans, not illustrated, are

provided to carry the loadlncident to forcesacting axially of the shaftII in either direction and a suitable flexible connection 30 is providedbecarry electric induction coils 36 and 38' respectively disposed.between the poles of respective permanent marmats'lfl and 42 rigidlysupported on the plat or base 28. so that any lateral movements in 1,-

.lebearings 20 and 22 will induce coaxial with the shaft 82 and sup insuitable bearings and ill mounted one. or machine base carries a ierallyindicated at til, adapted to enclose the propeller ill. The shroud twoseparable L and by some suitable s us such as 3 extending through ivecircum l portions, One a -1.1 1e portion is provided with a concentricaperture 3 through ich the propeller supporting shaft extends v thsufficient clearance so that the shal u will not strike the edge ofopening it when the propel er vibrates the maximum amount per mitted thebearings 21) and 122.

While simplified form of propeller enclosing shroud has beendiagrammatically illustrated, this shroud may take any desired orconvenient form and suitable means for supporting the shroud andsecuring the two portions together so that they can be easily separatedfor the purpose of mounting and dismounting the propellers therein, aybe provided without in any way exceeding the scope of the invention.

The shaft 54 is driven by a motor 12 which is preferably provided with aspeed regulating control H in addition to the switch 16 for making andbreaking the motor circuit. The motor I! may also, if desired, beprovided with a speed control device '18 in addition to the switch 80for making and breaking the circuit of this motor.

with this arrangement, when the propeller is enclosed in the shroud 62and the shroud is rotated with the propeller at approximately the samespeed, the aerodynamic effects of the propeller are eliminated and itmay be considered that any vibrational eilects caused by the pro pellerare due to defects in the mass balance of the propeller, After thedefects in the mass bal ance have been corrected and the propellerrotates with the shroudwithout producing substan tial vibration, theshroud may be removed from the propeller and any vibrational efiectsthen produced by the propeller may be assumed to be caused byaerodynamic defects which defects may then be corrected by properlyadjusting the angles of the various propeller blades, usually a suitableadjustment of one blade of a two or three bladed propeller beingsufilcient to substantially eliminate these vibrational effects. Whenboth of the above indicated corrections have been made the propellershould be in a condition of substantially perfect mass and aerodynamichal ance.

In the illustrated apparatus for quantitatively and positionallyindicating the propeller -o"* one form of device marketed by the MachineCompany of Medisom "Wisconsu A, for n easurin out of balance ing bodiesis utilizes. of "he theory ope iii

had to t reference may b" published by t their catalogue Elaar W79. itis to be undo so detecting apparatus iliusti'e L I \1 scribed is shownonly for the purpose or dlscl the invention and that any other suitableapparatus for this purpose may be substituted for the one so illustratedand described without in any way exceeding the scope of the invention.

in determining the magnitude and position or the effective massunbalance oi the propeller the following procedure may be followed:

The movable field or stator oi the measuring generator 56 i moved tozero degree angular position. so that the zero degree coil d2 of thisgenerator and the zero degree coil of the compensating generator 52 willproduce a current and a voltage, respectively, exactly phase with eachother. The switches '88 are post tioned in contact with the terminals 96and 92 so that, when the propeller ill is rotated, electric currentimpulses from the electrical pick-up will be fed the measuring systemposition of the switches 36 and 88 also places the compensatinggenerator 52 in the electrical circuit so that the voltages produced bythe stator coils of this generator will be in series with the voltagesgenerated in the electric pick ups. The movable elements 94 and 96 ofthe respective potentiometers 98 and I00 will now be moved to theposition indicated by a at the opposite ends of the connector II, theswitch I02 will be moved into contact with the terminal I04 and theswitch I 06 will be moved into contact with the terminal 96, switch I22will be closed to terminal I24, switch H4 will be closed to terminal H5and switch I I8 will be closed to terminal I20 so that the right handpick-up, generally indicated by the letter R and the pair of coils 86and I38 of the compensating generator will be placed in circuit with theamplifying device H2. With I06 connected to 96; positioning 96 at a willefiectively block out any electrical impulses generated by the left I28of the wattmeter I30, the wattmeter will indicate watts generated in thepiclpup R which are in phase with the current generated in coil 82 andwill indicate watts generated in the coil 84 of the compensatinggenerator 52. By now ad justing the potentiometer I32 which is in serieswith the coil 84, it is possible to make the resultant voltage zero andthus obtain a zero reading on the wattmeter i30. 'Under theseconditions, coil 34 of the compensating generator will be producing avoltage phase with the zero degree componentof the voltage output of theelectric pick-up 23B and of exactly the same magnitude.

After the above mentioned Zero reading of the been obtained switch inshould U ntact the terminals 533 and com- ,tt neter will now me thewatts potein reduce the watt is exactly compensated by the componentvolt-= ages generated the coils B4 and i558 of the compensatinggenerator 52 so that no current is passed through the movable coil tillof Lhe wattmeter to move the pointer I42. Now, it switches Hi2 and 5%are moved to contact the terminals 94 and M4 and a similar procedure isfollowed in adjusting the potentiometers M5 and across the coils I08 andH0, respectively, of the compensating generator, it will be possible toexactly compensate for the voltage being generated in the pick-up L. Themachine has now been adjusted so as to be electrically balanced. thatis, the compensating elements of the machine are so adjusted as tosimulate the condition which would exist if a propeller having perfeetmass and aerodynamic balance were mounted on the shaft Now, the shroud82 is temporarily removed and an unbalance is introduced into the systemby adding a small weight to the propeller: in a we determined planeparallel to the plane of rotation or" the propeller and a known distancefrom the axis of rotation and in line with the zero degree rotationalposition of the propeller as determined by the location of the fieldcoils and rotor of the measuring generator 556. When the propeller isrotated with the shroud replaced this unbalance will cause additionalmovement of both pick-ups R and L and cause additional voltages to begenerated by these pick-ups in phase with the current generated in coil82 of the measuring generator 50. With the compensating generatorremaining in the circuit the switch I22 is again positioned to contactthe terminals I24 to conmeet the coil 82 of the measuring generator withthe fixed or current coils of the wattmeter and switch I02 is returnedto contact with the terminal I04 while switch IDS is returned to contactwith the terminal 96 to again place the right hand pick-up R in thecircuit. 'The wattmeter should now be made to read zeroby adjusting thepotentiometer III!) which will in effectutilize some of the voltageinduced by the pick-up L to ppose the voltage'of pick-up R. Under theseconditions the zero wattmeter readingindicates that a ufliclent portionof the outputof the pick-up L, which is in electrical opposition withthe pickup R, has been combined with the latter to com- "pletely nullifyit. Thus, there will be no indica tion in pick-up R of unbalance in theplane of the added weight. The unbalance first added is now removed anda second known unbalance is placed on the propeller at a known distancefrom the axis of rotation in a diiferent plane parallel to the plane ofpropeller rotation and in line with the zero degree rotational positionof the machine the shroud being removed for this operation and replaced.The wattmeter will now indicate unbalance in the plane of this newweight. By adjusting the variable potentiometer M the wattmeter readingmay now be changed or callbrated to read thi known unbalance in anydesired unit.

Similar nullification and calibration operations may be carried out forunbalance in the plane the first added weight by adjusting moving "chesand i at to contacts ta l and t lt, rectively, and the potentiometer tozero indication on the wattmeter of thc as produced by the newly addedweight. The cud is a ain removed from the propeller a1 second weight isremoved and the first Wei' 1 out position the pro ller the wattrneterltd to read this known unbalance in a desired correction unit.

The'inachinc has now'heen completelycali brated and if both of the trialweights are removed and the shroud replaced around the pro peller andthe switches 86 and 38 move into contact with the terminals M8 and Mt,respectively, and the switches lid and lid moved into contact with theterminals Hi2 554, respectively, and the propeller rotated, thecompensating generator 52 will be taken out of the circuit and it willhe possible to exactly determine the amount and angular position of theoriginal unbalance in the propeller in the planes of the added trialweights. For example, in order to find the effect of initial unbalancethe plane of the first trial weight, switches and iilfi are moved tocontact the terminals lll 'l and as, respectively, and the switch tilt.is moved to contact the terrninals to connect the coil ltd of themeasuring generator with the wattmeter I30. If now a reading is observedon the wattmeter the stator of the measuring generator is rotated untila zero reading on the wattmeter is obtained. The angular positionindicated by this changed position of the stator of the measuringgenerator will indicate directly the angular position oi the unbalancein the selected plane of correction. If switch H2 is now thrown tocontact the terminals 524 and connect the coil 82 with the wattmeter thecurrent and voltage introduced to the wattmeter will be in phase and thewattmeter will indicate directly in the selected units of cormotion theamount of unbalance to be compenthe position determined duringcalibration with the second trial weight, the stator of the measuringgenerator may be again moved to give a zero the potentiometer isadgusted to cause Wattmeter reading thereby indicating the angularposition of the unbalance in the other plane of correction. Changing theposition of the switch I22 to now connect the coil 82 of the measuringgenerator with the wattmeter will indicate the amount of correction tobe made in this other plane of correction in the selected units ofcorrection.

After the indicated mass corrections in the two planes'have been thusdetermined, the shroud will be removed and weight added to thepropell'er of the indicated amount and at the indicated positions or, insome cases, weight may be removed from the propeller the indicatedamounts and at positions diametrically opposite the locations in whichweight would be added according to the first method.

After the mass unbalance of the propeller has been corrected with thepropeller rotating in the rotating shroud the shroud is removed and thepropeller rotated independently. By a repetition of above recess theapparatus may 5 their: unbalance by ""acteristics such as the plishedthe propeller will be-in complete mass and aerodynamic balance.

While a particular method 4nd apparatus has been hereinabove describedand a suitable apparatus has been diagrammatically illustrated for thepurpose of disclosing the invention, it is to be understood that theinvention is not limited to the particular method and apparatus soillustrated and described, but that such changes in the nature andsequence of the various steps of the improved method and in theconstruction and arrangement of the various parts of the apparatus maybe resorted to as come within the scope of the sub-joined claims.

Having now described the invention so that others skilled in the art mayclearly understand the same, what it is desired to secure by LettersPatent is as follows:

1. Apparatus for determining the mass and aerodynamic unbalance ofrotating bodies having aerodynamic characteristic comprising, means forindicating the magnitude and angular location of disturbing forcesincident to both mass and aerodynamic unbalance at variable rotativespeeds, and means for eliminating aerodynamic effects at any rotativespeed including rotatable means enclosing said rotating body androtating at substantially the same speed and in the same direction assaid body for rotating the air surrounding said body to therebysubstantially eliminate relative movement between the propeller and thesurrounding air while the disturbances due to mass unbalance are beingmeasured.

2. Apparatus for determining the mass and aerodynamic unbalance ofrotating bodies having aerodynamic characteristics comprising, means forindicating the magnitude and angular location of disturbing forcesincident to both mass and aerodynamic unbalance, and means comprising ashroud enclosing and rotatable with said rotating body at the same speedand in the same direction therewith for eliminating aerodynamic effectsby rotating the air surrounding said rotating body while thedisturbances due to mass unbalance are being measured. 7

3. Apparatus as set forth in claim 2 in which said shroud is separableand removable from its position enclosing said rotating body.

4. The method of balancing aeronautical propellers which comprises,eliminating aerodynamic effects from the operation of said propeller bycausing the body of air immediately surrounding the propeller to rotatewith the propeller at the same speed and in the same directiontherewith,

measuring the disturbing Iorces due to mass unbalance of the propellerwhile rotating with said aerodynamic effects eliminated, correcting themass unbalance by a re-distributionrof the pro peller mass, restoringthe aerodynamic eiiects, measuring the disturbing forces of said masscorrected propeller while rotating with said aerodynamic efiectsrestored, and correcting the aerodynamic unbalance of said propeller bychanging the aerodynamic effect of at least one of the propeller blades.

5. The method of balancing aeronautical propellers which comprises,eliminating aerodynamic effects from the operation of said propeller byenclosing said propeller in a hollow shroud rotatable with the propellerat the same speed and in the same direction therewith, measuring thedisturbing forces due to mass unbalance of the propeller while rotatingtherein, correcting the mass unbalance by a re-distribution of thepropeller mass, restoring the aerodynamic efiects by removing saidpropeller from said enclosing shroud, measuring the disturbing force ofsaid mass corrected propeller while rotating with said shroud removed,and correcting the aerodynamic unbalance of said propeller by changingthe pitch angle of at least one of the propeller blades.

6. The method of balancing aeronautical propellers which comprises,rotating said propeller within a shroud rotating withsaid propeller atthe same speed and in the same direction therewith, measuring thedisturbing forces due to mass unbalance of the propeller while rotatingwithin said rotating shroud, correcting the mass unbalonce by are-distribution of the propeller mass, removing said shroud, measuringthe disturbing forces or said mass corrected propeller while rotatingwithout said shroud, and correcting the aerodynamic unbalance of saidpropeller by changing the aerodynamic effect of at least one of thepropeller blades.

7. Apparatus for determining the mass and aerodynamic unbalance ofrotating bodies having aerodynamic characteristics comprising, means forindicating the magnitude and angular location of disturbing forcesincident to both mass and aerodynamic unbalance, and means comprising a.shroud enclosing said rotating body and rotatable therewith at the samespeed and in the same speed and in the same direction for eliminatingaerodynamic eifects while the disturbances due to mass unbalance arebeing measured and a. separate support and a controllable speed drivefor said shroud.

ERLE MARTIN.

